Surface treatment for articles of magnesium and alloys thereof



Patented Oct. E9, 1943 SURFACE TREATMENT FOR ART ICLES F MAGNESIUM ANDALLOYS THEREOF William S. Loose, Midland, Mich" assignor to The' DowChemical Company, Midland, Mich, a corporation of Michigan No Drawing.Application November 14, 1938, Serial No. 240,343

7 Claims.

The invention relates to methods of providing a protective coating uponarticles of magnesium and its alloys.

It has been proposed to produce protective coatings upon articles ofmagnesium and its alloys by immersion in an aqueous solution of asoluble fluoride, such as hydrofluoric acid, potassium fluoride, and thelike, the article being preferably made the anode, whereby a deposit orfilm of magesium fluoride is formed on the article. I have foundthat,when this process is carried out with acid fluoride solutions, thecoatings produced are subject to considerable deterioration whichpermits corrosion to occur of the underlying metal. This is due,according to my observations, to the presence in the deposited coatingof dilute hydrofluoric acid formed by the rinsing opera-' tion followingthe treatment and this acid readily attacks magnesium. I have now foundthat by treating the article with an aqueous solution containing asoluble alkali subsequent to the treatment in a soluble fluoride bath.and before rinsing that greatly improved results are obtained as regardsthe resistance to corrosion of the protective coating. The invention,then, consists of the method hereinafter fully described andparticularly pointed out in the claims.

In carrying the invention into effect the article is treated with anaqueous solution, as for example, one containing a fluoride having anacid reaction, whereby there is produced a coating of the charactergenerally known as a fluoride coating. For this purpose a solution ismade containing hydrofluoric acid or a soluble compound containing freehydrogen fluoride, as for example, by dissolving in water a bifluoride.Besides hydrofluoric acid numerous bifluorides are available that aresuitable for the purpose, such as sodium,

potassium, and ammonium bifluoride. Similar solutions can be made bydissolving a neutral fluoride in water and adding hydrofluoric acid. Thecomplex acid fluorides may be used as, for example, fluosilicic acid(HZSiFG) and hydrofluoboric acid (HBF4) The concentration at which toemploy the fluorine compound may .vary over a wide range from as littleas about 0.25 per cent by weight of the equivalent of HF up to thesaturation point of the compound employed. A preferred range ofconcentration is between about to the boiling point of the solution, thehigher the temperatune employed the faster the initial coating isproduced. The time of such treatment may be from one-half minute to 10minutes or more depending upon the temperature and the thickness towhich it is desired to build up the coating. If desired, the article maybe anodically electrolyzed while being treated in the solution, wherebythe production of the coating is facilitated'. A suitable voltage toemploy is between and 100, using amagnesium cathode.

After having produced the fluoride coating in the manner described, thearticle is subjected to the action of an aqueous solution containing asuitable alkali whereby any residual fluorine acidity in the pores ofthe coating previously produced in neutralized, such step being takenwithout rinsing off the previously used solution. For

produce insoluble fluorides, such as calcium hydroxide, strontiumhydroxide, etc., by reaction with a soluble fluoride in a concentrationof from about 0.1 of apercent to 30 per cent or more in water solutiondepending upon the solubility of the selected alkali. In general, Iprefer to use a concentration of about 10 per cent by weight and atthe-boiling temperature when an alkali metal alkali is used. The processmay be carried out, however, at any ordinary temperature, the higher thetemperature the less the time required. In general, the duration of thecontact between the solution and the article should be from one-half tofive minutes or more. In a preferred embodiment of the invention, I mayuse a saturated solution of calcium hydroxide or lime water for thesecond step, by immersing the article in it for 3 minutes at roomtemperature.

If desired, the article may be anodically electrolyzed in the alkalinesolution. This has the efiect generally of enhancing the resistance tocorrosion of the flnishedarticle. A potential of about 15 volts or moremay be used and preferably about 50-60 at a temperature of about 50 C.although other temperatures may be used. The bath is suflicientlyalkaline for this anodic treatment when the pH is about 12 or abovewhich is readily obtainable by adding from 2 to 6 per cent of sodiumhydroxide to water.

The following examples are further illustra tive of the invention:

Example 1 A similar magnesium alloy article was immersed in a 52 percent solution of hydrofluoric acid at 45 C. and anodized at a potentialof 60 volts for 12 minutes. The article was then removed from thesolution and subjected to the actionof a per cent solution of sodiumhydroxide for 1 minute at 20 C. The article was then rinsed and dried.

Example 3 A similar magnesium alloy'article was immersed in a 30 percent solution of hydrofluoric acid at 30 C. for 12 minutes. The articlewas then removed from the solution and subjected to the action of anaqueous solution containing 5.0 per cent of sodium hydroxide at roomtemperature for 5 minutes. The article was then rinsed and dried.

As an illustration of the improvement resulting from the treatment withthe alkali solution subsequent to the treatment in a solution of asoluble fluoride, comparative data was obtained of the resistance tocorrosion as measured by the usual alternate immersion test in 3 percent salt solution at 95 F. The percentage of the surface area which wasattacked was measured and this was used to express the amount ofcorrosion.

The articles treated as described in the above examples and subjected tothe test for corrosion resistance showed an average per cent surfaceattack in a week of approximately 9 per cent. In comparison with this,the average per cent surface attack of similar articles treated inconventional manner with a fluoride solution alone and then similarlytested was approximately 100 per cent. Treatment of similar magnesiumarticles only in an alkaline solution, such as those used above, doesnot produce a signincant amount of protective coating.

It is to be understood that the method may be used with other magnesiumalloys than those illustrated.

Other modes of applying the principle of my invention may be employedinsteadof those explained, change being made as regards the methodherein disclosed, provided the step or steps stated by any of thefollowing claims or the equivalent of such stated step or steps beemployed.

I therefore particularly point out and distinctly claim as my invention:

1. A method of treating articles of magnesium and its alloys comprisingsubjecting the article to the action of an aqueous solution consistingessentially of acid fluoride, and thereafter, without rinsing off thefluoride solution, subjecting the treated article to the action of anaqueous solution of an alkali.

2. A method, according to claim 1 in which the acid fluoride is selectedfrom the group consisting of hydrofluoric acid, ammonium bifluoride, analkali metal bifluorides. I

3. A method of treating articles of magnesium and its alloys comprisingsubjecting the article to the action of a solution consistingessentially of water and from about 0.25 to per cent of HF, andthereafter, without rinsing oil the fluoride solution, rinsing thetreated article in an aqueous solution of a soluble hydroxide.

4. A method, according to claim 3, in which the soluble hydroxide is oneproducing an insoluble fluoride by reaction with a soluble fluoride.

5. A method, according to claim 3, in which the soluble hydroxide iscalcium hydroxide.

6. A method of treating articles of magnesium and its alloys comprisingsubjecting the article to the action of an aqueous solution consistingessentially of a soluble acid fluoride and then, without rinsing off thefluoride solution, anodically electrolyzing the treated article in anaqueous solution of an alkali. V

'7. A method, according to claim 6, in which the alkali is an alkalimetal hydroxide.

WILLIAM S. LOOSE.

